Since the first multiple hearth incinerator for the oxidation of sludge was installed in the United States in 1935, many municipalities have used multiple hearth incineration as a thermal reduction process prior to final disposal of waste. Since 1935, over three hundred of these units have been installed in the United States. These furnaces have handled a large variation in feed quality and loading rates.
In the conventional multiple hearth unit, the sludge cake enters the top of a cylindrical furnace and is moved from hearth to hearth through the furnace by a series of rabble arms extending from a center shaft along the full height of the furnace. Minimally, three functional zones exist in the furnace: (1) the upper part of the furnace evaporates the water contained in the sludge waste, resulting in a flue gas exit temperature of between 800.degree. F. to 1000.degree. F., (2) combustion occurs in the middle sections of the furnace with temperatures typically in the range of 1400.degree. F. to 1600.degree. F., and (3) the resulting ash is then cooled in the bottom of the furnace prior to being discharged. Presently, close to 75% of these incinerators have been closed down due to the following reasons: (a) increased sludge production, (b) a lowering of cake solids due to secondary treatment, (c) the ever increasing price of energy, and (d) the necessity of afterburning in order to meet air emission standards. All of these requirements are causing the cost for conventional incineration to dramatically increase and are forcing new incineration technology and process development.